Cordless telephone TDMA radio systems include a number of fixed base stations and a set of portable handsets (portables). The portables communicate digital data to and from the base stations on TDMA radio channels. In TDMA, each base station receives data from a given portable in short bursts followed by intervening periods during which no data is received. During the intervening periods, the base station may be communicating with other portables.
It is well known that TDMA radio channels are subject to the impairment of multipath fading. Such fading is statistically independent for a pair of properly spaced antennas at a base station receiver for example. At any instant in time, one of the two antennas may be performing better than the other, and ideally a signal should be received through the better performing antenna. During the period between the reception of bursts on a particular channel, the physical channel between the portable and the base station may change as a result of the movement of the portable itself, or as a result of the movement of some other object. As a result of this, it is possible that the antenna which was better for the previous burst is no longer the better for the current burst. With selection diversity, at the beginning of each burst a decision is made as to which antenna is to be used to receive that entire burst.
Standards exist which stipulate the data structure of a TDMA burst. In the DECT (Digital Enhanced Cordless Telecommunications) standard for example, GMSK (Gaussian minimum shift keying) is used to transmit bursts having a structure which includes a set of preamble bits used for timing recovery and a sync word used for slot synchronization. Following the sync word are the information bits. With selection diversity, in order to properly recover the information bits of a given burst, a decision as to which antenna to use for that burst must be made early enough in the reception of the burst that timing and synchronization can still be recovered before the information bits start to arrive. Another TDMA standard is the PWT (personal wireless telephony) standard. PWT is the North American variant of DECT, and utilizes .pi./4 DQPSK modulation instead of GMSK.
One way to make the antenna selection decision is to have two receivers receiving the signal simultaneously, each receiver having its own antenna. Performance measurements for the two receivers are made simultaneously and the receiver output which is better is selected.
A much more economical approach is to use a single receiver having two alternatively connectable antennas and to perform both antenna measurements one after the other and then choose the better antenna for the remainder of the signal. Typically, in order to determine which antenna to use, the first antenna is connected early in the reception of the burst and an estimate of the performance is made for that antenna. Then the second antenna is connected and an estimate of the performance is made for that antenna. If the second antenna is performing better than the first antenna then the second antenna is used to receive the burst. Otherwise, the first antenna must be reconnected to receive the remainder of the burst.
A problem with existing TDMA systems is that there is not enough time to make the performance measurements for the two antennas, decide on which antenna to use, recover timing, and recover synchronization before the information bits start to arrive.
Some prior art patents simply neglect to deal with this problem. Published PCT application WO 96/08089 published on Jun. 27, 1995 to Mogensen discloses a selection diversity system for use in a TDMA environment in which the quality of the signal received at the receiver on each of two antenna branches is measured at the beginning of each burst, and the antenna with the better quality is selected. No mention is made of the requirement to recover timing and synchronization. In Mogensen, it is conceded that some of the data will be lost because of the antenna measurements, this lost data being recovered through the use of interleaving and efficient coding. This system is not applicable to DECT or PWT systems as they do not employ interleaving or coding.
Published PCT application WO 96/02984 published on Feb. 1, 1996 to Akerberg discloses a method of selecting a given antenna in a TDMA system by making antenna performance measurements during the preamble period of a DECT TDMA time slot. By using up the preamble bits to make the performance measurements, there is a shortened period during which to recover timing and synchronization. Akerberg does not address the problems of recovering the timing and synchronization of the signal before the arrival of the information bits.
In a recent modification of the DECT standard, an optional extended preamble is added which provides 16 extra bits to the beginning of each time slot during which the performance measurements of two antennas can be made. This leaves the normal preamble and sync words available for the recovery of timing and synchronization. A problem with this is that a different data format at the portable must be used, and thus existing portables which do not provide the extended preamble can not be used to provide selection diversity at the base station.